The present application relates to electric power conversion, and more particularly to buck-boost converter circuits, methods and systems which can convert DC to DC, DC to AC, and AC-AC, and are suitable for applications including line power conditioners, battery chargers, hybrid vehicle power systems, solar power systems, motor drives, and utility power conversion.
Numerous techniques have been proposed for electronic conversion of electric power from one form into another. A technique in common commercial usage for operating three phase induction motors at variable frequency and voltage off of fixed frequency and voltage utility power is the AC-DC-AC technique of the input diode bridge, DC-link capacitor, and the output active switch bridge, under PWM control, is shown in FIG. 3. This motor drive technique (“standard drive”) results in compact and low-cost motor drives, since no magnetic components are required and only six active switches are needed.
A number of difficulties exist with the standard drive, however. The input current, while nominally in phase with the input voltage, is typically drawn in pulses. These pulses cause increased electric losses in the entire electrical distribution system. The pulses also cause higher losses in the DC link capacitor. These losses reduce the efficiency of the drive, and also lessen the useful life of the DC link capacitor (commonly an Aluminum Electrolytic type), which has a limited life in an case. If the impedance of the source power is too low, the pulses may become so large as to be unmanageable, in which case it is necessary to add reactance in the input lines, which increases losses, size, cost, and weight of the drive. Also, the voltage available for the output section is reduced, which may lead to loss-producing harmonics or lower-than-design voltage on the output waveform when full power, full speed motor operation is called for.
Due to the fixed DC-link voltage, the output switches are typically operated with Pulse Width Modulation (PWM) to synthesize a quasi-sinusoidal current waveform into the motor, using the inductance of the motor to translate the high voltage switched waveform from the drive into a more sinusoidal shape for the current. While this does eliminate lower order harmonics, the resulting high frequency harmonics cause additional losses in the motor due to eddy current losses, additional IR (ohmic) heating, and dielectric losses. These losses significantly increase the nominal losses of the motor, which reduces energy efficiency, resulting in higher motor temperatures, which reduces the useful life of the motor, and/or reduces the power available from the motor. Additionally, due to transmission line effects, the motor may be subject to voltages double the nominal peak-to-peak line voltage, which reduces the life of the motor by degrading its insulation. The applied motor voltages are also not balanced relative to ground, and may have sudden deviations from such balance, which can result in current flow through the motor bearings for grounded motor frames, causing bearing damage and reduced motor life. The sudden voltage swings at the motor input also cause objectionable sound emissions from the motor.
The output switches used in this motor drive must be constructed for very fast operation and very high dV/dt in order to minimize losses during PWM switching. This requirement leads to selection of switches with drastically reduced carrier lifetimes and limited internal gain. This in turn decreases the conductance of each device, such that more silicon area is required for a given amount of current. Additionally, the switches must be constructed to provide current limiting in the event of output line faults, which imposes additional design compromises on the switches which further increase their cost and losses.
Another problem with the standard drive is that the DC link voltage must always be less than the average of the highest line-to-line input voltages, such that during periods of reduced input voltage (such as when other motors are started across-the-line), the DC link voltage is insufficient to drive the motor.
Yet another difficulty with the standard drive is its susceptibility to input voltage transients. Each of the input switches must be able to withstand the full, instantaneous, line-to-line input voltage, or at least the voltage after any input filters. Severe input transients, as may be caused by lightning strikes, may produce line-to-line voltages that exceed 2.3 times the normal peak line-to-line voltages, even with suitable input protection devices such as Metal Oxide Varistors. This requires that the switches be rated for accordingly high voltages (e.g. 1600 volts for a 460 VAC drive), which increases cost per ampere of drive.
The standard drive also cannot return power from the DC link to the input (regeneration), and therefore large braking resistors are required for an application in which the motor must be quickly stopped with a large inertial or gravitational load.
Modifications to the basic motor drive described above are available, as also shown in FIG. 3, but invariably result in much higher costs, size, weight and losses. For example, in order to reduce input current harmonics (distortion) and to allow for regeneration, the diode bridge may be replaced by an active switch bridge identical to the output switch bridge, which is accompanied by an input filter consisting of inductors and capacitors, all of which result in higher costs and drive losses. Also, as shown in FIG. 3, output filters (“sine filter”) are available to change the output voltage waveform to a sinusoid, but again at the expense of greater cost, size, weight, and losses.
AC-AC line conditioners are constructed in a similar fashion to the standard drive with input and output filters and an active front end, and also suffer from the above mentioned problems.
Other motor AC-AC converters are known, such as the Matrix Converter, Current Source Converter, or various resonant AC and DC link converters, but these either require fast switching devices and substantial input and/or output filters, or large, lossy, and expensive reactive components, or, as in the case of the Matrix Converter, are incapable of providing an output voltage equal to the input voltage.
The term “converter” is sometimes used to refer specifically to DC-to-DC converters, as distinct from DC-AC “inverters” and AC-AC “cycloconverters.” However, in the present application the word converter is used more generally, to refer to all of these types and more.
What is needed then is a converter technique which draws power from the utility lines with low harmonics and unit power factor, is capable of operating with full output voltage even with reduced input voltage, allows operations of its switches with low stress during turn-off and turn-on, is inherently immune to line faults, produces voltage and current output waveforms with low harmonics and no common mode offsets while accommodating all power factors over the full output frequency range, operates with high efficiency, and which does so at a reasonable cost in a compact, light-weight package.
DC-DC, DC-AC, and AC-AC Buck-Boost converters are shown in the patent and academic literature which have at least some of the aforementioned desirable attributes. The classic Buck-Boost converter operates the inductor with continuous current, and the inductor may have an input and output winding to form a transformer for isolation and/or voltage/current translation, in which case it is referred to as a Flyback Converter. There are many examples of this basic converter, all of which are necessarily hard switched and therefore do not have the soft-switched attribute, which leads to reduced converter efficiency and higher costs. An example of a hard switched 3 phase to 3 phase Buck-Boost converter is shown in FIG. 4, from K. Ngo, “Topology and Analysis in PWM Inversion, Rectification, and Cycloconversion,” Dissertation, California Institute of Technology (1984).
One proposed DC-AC Buck-Boost converter (in U.S. Pat. No. 5,903,448) incorporates a bi-directional conduction/blocking switch in its output section to accommodate four quadrant operation, with AC output and bi-directional power transfer. The input, however, cannot be AC, and it uses hard switching.